1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring composition which utilizes a coloring reaction of an electron donating coloring agent and an electron accepting color developer, a reversible thermosensitive recording material which uses the coloring composition and in which a colored image is repeatedly formed and erased by appropriately heating and cooling the reversible thermosensitive recording material, and a reversible thermal recording method using the recording material.
2. Discussion of the Related Art
A variety of thermosensitive recording materials are well known in which a colored image can be formed by a coloring reaction when an electron donating coloring agent is brought into contact with an electron accepting color developer while heat or the like is applied thereto.
The thermosensitive recording materials have the following advantages over other conventional recording materials:
(1) color images can be rapidly recorded by a relatively simple apparatus without using such complicated steps as developing and fixing; PA1 (2) color images can be recorded without producing noise and environmental pollution; PA1 (3) various color images, e.g., red, blue, violet and black, can be easily obtained; PA1 (4) image density and background whiteness are high; and PA1 (5) the manufacturing cost is relatively low. Because of these advantages, the above-described thermosensitive recording materials can be widely used, not only as a recording material for price labels in stores, but also as recording materials for copiers, printers for computers, facsimiles, automatic vending machines of tickets, video printers and measuring instruments. PA1 (1) a thermosensitive recording material which includes gallic acid in combination with phloroglucinol serving as color developers (Japanese Laid-Open Patent Publication No. 60-193691); PA1 (2) a thermosensitive recording material which includes phenolphthalein or Thymolphthalein serving as a color developer (Japanese Laid-Open Patent Publication No. 61-237684); PA1 (3) a thermosensitive recording material which includes an uniform solid solution of a coloring agent, a color developer and a carboxylic acid ester (Japanese Laid-Open Patent Publications No. 62-138556, 62-138568 and 62-140881); PA1 (4) a thermosensitive recording material which includes an ascorbic acid derivative serving as a color developer (Japanese Laid-Open Patent Publication No. 63-173684); and PA1 (5) a thermosensitive recording material which includes bis(hydroxyphenyl)acetic acid or a higher aliphatic amine salt of gallic acid serving as a color developer (Japanese Laid-Open Patent Publication No. 2-188294).
However, the coloring reaction of these thermosensitive recording materials is irreversible and accordingly these recording materials cannot be repeatedly used.
Several thermosensitive recording materials which can reversibly form and erase an image have been proposed. For example, they are as follows:
Some of the present inventors, jointly with others, have proposed a reversible thermosensitive coloring composition which includes a coloring agent such as a leuco dye and a color developer such as an organic phosphate compound, an aliphatic carboxylic acid compound or a phenolic compound each of which has a long-chain aliphatic hydrocarbon group, and a reversible thermosensitive recording material using the coloring composition (Japanese Laid-Open Patent Publication No. 5-124360). The reversible thermosensitive recording material can stably repeat image formation and image erasure by being appropriately heated and cooled, and the image-recorded state or the image-erased state can be stably maintained at room temperature. In addition, Japanese Laid-Open Patent Publication No. 6-210954 has disclosed a reversible thermosensitive recording material which includes a specified phenolic compound having a long-chain aliphatic hydrocarbon group serving as a color developer.
Thus, reversible thermosensitive recording materials have been proposed; however, these recording materials do not have requisite properties such as quick erasability and good preservability when they are preserved under various environmental conditions or when light is irradiated thereto.
Because of these reasons, a need exists for a reversible thermosensitive recording material which has good image formation/erasure ability and rapid erasability, and particularly has good preservability even when preserved under high temperature conditions.